This invention relates generally to electrical switch gear and more particularly to switch gear such as high-voltage enclosed switchboards and vacuum load-break switches having high dielectric strength, high degree of safety, and moreover the possibility of being miniaturized.
A typical example of a known switch gear has an outer housing in the form of a sealed box structure of steel plate, the interior of which is divided into a plurality of chambers or compartments by a plurality of partition walls. Within these compartments, various items of electrical equipment such as a disconnector, a circuit breaker, a current transformer, and a lightning arrester are installed. In general, a box structure thus accommodating such electrical equipment contains air in its unoccupied spaces. Recently, however, sulfur hexafluoride gas has been sealed in as an insulation gas in a part of the box structure to improve the insulating characteristic between the various items of electrical equipment and also between metal members for grounding and thereby to achieve miniaturization of the entire switch gear.
In a switch gear of such known structural organization, problems as enumerated below have been encountered.
1. First, the outer box structure is required to be gas tight in order to prevent leakage of the sulfur hexafluoride (SF.sub.6) gas sealed in therewithin. For this reason not only does its fabrication (welding and assembly) require much labor, but a monitoring device for monitoring the interior pressure after sealing in of the gas is necessary.
2. If, in order to seal in SF.sub.6 gas within the box structure, the air in the interior thereof is to be first evacuated, it will be necessary to reinforce the box structure that it can withstand the difference between the interior and exterior pressures. This will entail an increase in fabrication cost. On the other hand, if the air is replaced by the SF.sub.6 gas by a forced circulation method, structural reinforcement will not be necessary, but labor and time will be required for carrying out the replacement.
3. Since the box structure must be a sealed vessel, the internal pressure will rise in the event of an interior short circuit due to some cause. This will give rise to the possibility of an explosion which would be extremely dangerous to the maintenance personnel.
4. For inspection of the equipment within the box structure (for example, inspection of the degree of wear of the contact points of the circuit breaker), discharging and recharging of the insulation gas before and after unsealing is necessary, whereby much time is required for maintenance and inspection.